1. Field of the Invention
The present invention relates to a polarizing plate for use in a liquid crystal display, an optical communication, optical sensors, polarizing sunglasses or the like and to its manufacturing process.
2. Description of the Prior Arts
As a linear polarizing plate heretofore used in a liquid crystal display or the like, a linear polarizing film using iodine or dye is well known. In the film, by stretching a base film, dichroic materials of iodine or dye are oriented to obtain a polarizing property. Therefore, at present, the base film is restricted to a material suitable for the stretching process, i.e., polyvinyl alcohol or the like.
Also, in a stretching process of the base film, it is necessary to control and adjust a temperature in the stretching process, a film quality, a stretching force, etc., strictly.
To solve the problem, a publication of patent application examined and published No. Hei 8-27409 proposes a polarizing plate in which fine magnetic particles are arranged in a constant direction by a magnetic field in a polarizing layer and interconnected to form multiple bar-shaped elements. In the proposed polarizing layer, a direction of a major axis of the bar-shaped elements shows an absorption axis of light and a direction perpendicular to the major axis direction shows a transmission axis. Thus, in the patent, it is reported that a light through the polarizing layer is polarized and the direction of the transmitted light is in the same direction perpendicular to the major axis direction of the bar-shaped elements.
However, the bar-shaped elements disclosed in the publication No. Hei 8-27409 are formed in bar configurations by interconnecting plural substantially spherical particles by means of magnetic field. It is remarkably difficult to control the number of the magnetic particles constituting the bar configurations. Therefore, the configurations of the bar-shaped elements cannot become uniform (an aspect ratio of a length of the major axis to a length of the minor axis is not constant), and an extinction ratio should be low. It is also difficult to adjust the aspect ratio of the bar-shaped elements in accordance with a wavelength of a light for use. Also, in the interconnected bar-shaped elements, the magnetic particles can optionally change directions at each interface of the particles. Therefore, it is difficult to align all the bar-shaped elements orderly in the same direction to the direction of magnetic field. Further, assuming that the polarizing effect of the bar-shaped elements described in the publication No. Hei 8-27409 is due to a principle that a light which has a certain direction of an electric field was absorbed because electrons in the magnetic particles vibrates in the same direction of the electric field, then in case of No. Hei 8-27409, the extinction ratio is rather small because electrons cannot move beyond the boundary of the magnetic particles. A light which has an electric field in the same direction of the major axis of the magnetic particles, is not absorbed enough because electrons cannot move smoothly at the boundaries. From the above, it is difficult to prepare a polarizing plate with a high extinction ratio in the method disclosed in the publication No. Hei 8-27409.
On the other hand, a publication of patent application laid-open No. Sho 60-212706 proposes a method of manufacturing a polarizing plate by aligning dichroic materials using a magnetic field or an electric field. Particularly, as the dichroic material, a liquefied tin chloride is used in an embodiment. Further, it is described that metallic salts, metallic crystals, non metallic crystals or the like are other preferable materials. However, the polarizing plate has been increasingly requested to have a high level of the extinction ratio or other properties. It cannot be said that the prior arts in the patent of No. Sho 60-212706 reach a satisfactory level. Further, the use of bar-shaped particles is not described, and it can be said to be difficult to obtain a high extinction ratio. Additionally, the dichroic materials (tin molecules or crystals) described as the embodiment in the publication No. Sho 60-212706 are just existed to be aligned using a magnetic field on a transparent glass plate, but are not fixed in a laminated cured medium. Therefore, it can be supposed that in the process of forming a protective layer, the substances may collapse because of some physical external forces, e.g., a pressure applied on the crystals or the molecules. Further, the aforementioned proposals have a problem that the polarizing property cannot be easily optimized in accordance with the wavelength of the light for use.
A polarizing plate used in optical communication is disclosed in a publication of patent application laid-open No. Sho 56-169140. This is a polarizing glass which is constituted by dispersing bar-shaped silver halide and silver particles into glass. Among the components dispersed in the glass of prior art, the silver particles are considered to mainly exhibit a dichroism and contribute to the polarizing effect.
Also, in a technical document, Proc. SPIE Int. Soc. Opt. Eng., Vol. 1166, pp. 446-453, it is disclosed that by increasing a length of a major axis of bar-shaped silver particles, a wavelength at which a high extinction ratio can be obtained is shifted to longer wavelength. Thus, it is suggested that the wavelength at which the high extinction ratio can be obtained can be controlled by a particle aspect ratio.
The aspect ratio of the bar-shaped silver particles which are disclosed in the publication No. Sho 56-169140 is determined by pressures and heat treatment conditions in elongating process of glass and reduction process. However, it is very difficult to control the conditions. The distribution of the aspect ratio cannot be adjusted optionally. Usually, the distribution of the aspect ratio tends to be a normal distribution.
As a related art of the publication No. 56-169140, a publication of J.P. Patent No. 2578377 discloses a polarizing glass which similarly uses silver halide or silver particles and exhibits a high extinction ratio in a relatively broad wavelength region. According to the publication, by performing a burning process under pressure and a reducing environment instead of performing under atmospheric pressure, an effective bandwidth in which the polarizer shows a high extinction ration can be broadened. Although the polarizing glass exhibits the high extinction ratio in the relatively board band as aforementioned, its manufacturing process disadvantageously requires a large number of steps. Further, in analogy with the polarizing glass disclosed in the publication No. Sho 56-169140, it is very difficult to control pressures and thermal treatment conditions for elongating the glass and reduction process. It is, therefore, difficult to prepare a large quantity of uniform polarizing glasses inexpensively.
Also, as aforementioned, the publication No. Hei 8-27409 proposes the polarizing plate in which the magnetic particles are arranged in the constant direction by the magnetic field in the polarizing layer and interconnected to form the multiple bar-shaped elements. The bar-shaped elements are formed in bar configurations by interconnecting multiple substantially spherical elements through a magnetic field orientation. Therefore, it can be said that it is remarkably difficult to control the number of the magnetic particles constituting the bar configurations. Since the bar-shaped elements cannot be uniformly formed, it is remarkably difficult to control the aspect ratios and the distribution thereof. Also in the bar-shaped elements, since the magnetic particles can freely change directions at each interface of the particles, it is difficult to align all the bar-shaped elements orderly in the same direction to the direction of the magnetic field. Therefore, it is difficult to produce a polarizing plate showing high extinction ratio in the broad wavelength region, uniformly.
The publication No. 2578377 proposes the method of manufacturing the polarizing glass by dispersing and depositing the Ag particles in a glass matrix. According to the prior art, the manufacturing process of the polarizing glass requires a large number of steps, such as melting of glass, thermal treatment, stretching, grinding/polishing, reduction and burning processes. Further, each process requires a remarkably complicated operation. Detailed descriptions about preparing process of the polarizing plate are as follows. First, a glass containing silver halide particles are stretched to make the silver halide particles ellipsoid and to align the particles in the stretched direction. Then, after the polishing process, the glass containing silver halide particles are reduced and burned in a hydrogen atmosphere to deposit the Ag particles in the glass and, finally, the polarizing glass is obtained.
However, in the manufacturing process of the polarizing glass disclosed in the publication of patent No. 2578377, the aspect ratio of the deposited Ag particles cannot be adjusted easily. Also, the manufacturing process includes multiple steps and is not suitable for mass production. Further, the manufacturing method is not appropriate for a large-sized polarizing glass. Therefore, it is difficult to supply a large quantity of large-sized polarizing plates inexpensively.